Re: Need help doing a jmp rather than a call

[Sofiane Akermoun <akersof@xxxxxxxxx>]

Do you really need assembly to perform this?

Why func2 doesn't return a pointer on func4 to the caller func1?
This way func2 do this job, and return to func1 a pointer on a
function to call func4.
A very little, simple and naive simple:

#include <stdio.h>

void func4(char *str)
{
    printf("%s", str);
}

void (*func2(void))(char *)
{
    //we can imagine here some work on func2 that will decide what
function  to return
    return func4;
}

void func1(char *str)
{
   //Here func2 return func4 pointer and
   //it is used ny func1 to call func4 function with "str" argument
   func2()(str);
}

int main(int argc, char *argv[])
{
    func1("Hello world!");
    return 0;
}

It is what you need?

kind regards,
Sofiane Akermoun

2013/11/9 Blake McBride <blake@xxxxxxxxxxx>:
> I am trying to get a simple piece of x64 assembly working on 64 bit linux
> and a Mac.  I am using GCC.
>
> Let's say I have 4 (C language) functions.  Function fun1 calls fun2, fun2
> calls fun3, and fun3 calls fun4.  I need fun 4 to operate and run as if it
> was called directly from fun1.  So, fun4 should see the arguments passed to
> fun2, and when fun4 returns it should return diretly to fun1 as if it was
> called by fun1.
>
> Basically, this is an OO language that uses fun2 to calculate what fun4 is.
> fun3 is used to manipulate the stack so that when fun4 starts up it thinks
> it was called directly from fun1.  fin1, fun2, and fun4 are plain C code.
> fun3 performs the magic.  I have been doing this easily on many different 32
> bit machines for years but I haven't been able to get it going on 64 bit
> machines.
>
>
> Typically fun3 would:
>
>    1. create a new stack frame
>    2.  call fun3
>    3.  pop local call frame
>    4.  return
>
> What I need fun3 to do is:
>
>    1.  either pop its stack frame or don't create one
>    2.  pop the stack frame from fun2
>    3.  jump to fun4
>
> Then fun4 will start executing as if it was called from fun1.  I have sample
> (errant) C code for the entire process.  It will all work fine if fun3
> (_jumpToMethod) is rewritten in assembler.
>
> (What I did in the past was compile the C code for _jumpToMethod into
> assembly code, modify the code, and then use that assembly code.)
>
> Here is the C code for fun1, fun2, and fun3:
>
> #include <stdio.h>
>
> char    *obj = "Some object pointer";
>
> char    *GenObj = "Some Generic Object Pointer";
>
> typedef int     (*ofun)();
>
>
> int     Method(char *self, int a, int b, int c)  /*  fun4  */
> {
>         printf("Method reached with args %s %d %d %d\n", self, a, b, c);
>         return a + b + c;
> }
>
> ofun    FindMethod(char *obj, char *gen)
> {
>         return Method;
> }
>
> GenericFunction(char *self, ...)  /*  fun2  */
> {
>         _jumpToMethod( FindMethod(self, GenObj) );
> }
>
> main(void)  /* fun1  */
> {
>         printf("Method is at %lx\n", (long unsigned int) &Method);
>         /* both calls to Method should look alike to Method  */
>         int     r = Method(obj, 1, 2, 3);
>         printf("Value returned from GenericFunction = %d\n", r);
>         r = GenericFunction(obj, 1, 2, 3);
>         printf("Value returned from GenericFunction = %d\n", r);
>         return 0;
> }
>
>
> -----------------------------------------------------------------
>
> Here is the C code for _jumpToMethod (fun3)
>
>
> void    _jumpToMethod(void (*function) (/* ??? */))
> {
>
> /*      pop_this_stack_frame;   */
>
> /*      pop previous (generics) stack frame  */
>
>         (*function)();  /*  must be changed to jump instruction   */
> }
>
>
>
> I think this would be easy for someone who knows this assembly language.  I
> have spent about 5 hurs on it and I am lost as I can be.  Your help is
> greatly appreciated!
>
> Thanks!
>
> Blake McBride
>
>
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-- 
Sofiane AKERMOUN
akersof@xxxxxxxxx
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